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Southern California airports are some of the busiest in the nation. On average, the US sees one mid-air collision a month. 60 percent of the time they are fatal, like Monday's deadly plane collision, and many of them happen within five miles of an airport. Chuck Henry reports for the NBC4 News at 6 p.m. on April 30, 2013. (Published Tuesday, April 30, 2013)

The midair collision of two planes over Westlake Village this week points out that no matter how big the sky, it’s rarely wide open. And the skies over Southern California are among the most crowded in the country.

Part of the reason for that is the sheer number of airports in the Los Angeles area. From the biggest ones like LAX and John Wayne to smaller ones like Hawthorne and El Monte, there are 30 airports in the area.

From those airports are millions of takeoffs and landings. In 2009, Van Nuys Airport earned the title of “world’s busiest general aviation airport” with 400,000 takeoffs and landings that year.

The recession and the high cost of fuel has helped reduce that number, but the latest statistics available show there have been more than 61,000 takeoffs and landings in Van Nuys in the first three months of this year. That averages nearly 700 takeoffs and landings each day from a single Southland airport.

Both planes involved in Tuesday’s midair collision took off from Santa Monica Airport, where records show there were nearly 8,500 takeoffs and landings in January, with more than 8,700 in February.

A tremendous number of aircraft are in the sky at any one moment.

Larry Welk has flown planes and helicopters over California for more than 20 years, including the NBC4 helicopter.

“Airspace continues to get more and more dense,” said Welk, who added that LA airspace is “complex.” Pilots have to contend not only with substantial traffic, but other issues like terrain.

There have been notable midair collisions in the past five years in Southern California.

In May 2009, two planes collided over Long Beach, killing three people.

In January 2008, two planes hit each other over Corona, raining down debris over a strip of auto dealerships. Four people on board the planes were killed, along with one victim on the ground.

According to the Aircraft Owners and Pilots Association, Southern California rivals New York for the volume of air traffic. In New York, pilots complain about what they say can be a hazardous flight over the Hudson River, where nine people died in a 2009 collision between an airplane and a tour helicopter.

According to the National Transportation Safety Board, there have been 120 midair collisions since 1999, killing more than 160 people.

Welk said technology that can warn pilots even of small planes of approaching aircraft is minimizing the danger posed by midair collisions.